1. Field of the Invention
The present invention relates to the termination of line impedances. More specifically, the present invention relates to terminating line impedances by modifying a nominal impedance using digital impedance synthesis.
2. Present State of the Art
As technology increases around the world, more and more people are using computers and computer related hardware for entertainment, personal and business reasons. Computers also allow persons from remote locations to communicate with other remotely located individuals or businesses. These communications can occur over the internet, via satellite communication or over telephone networks.
As the need for global communication has increased, the technology which permits such communication has also advanced. The problem is that different countries have not agreed on any single standard. For example, virtually every country has a telephone system or network. Each telephone network, unfortunately, is designed a bit differently. This is evident from the electrical characteristics which partly define such systems. One important characteristic is the nominal or characteristic impedance. The characteristic impedance is usually defined in ohms, but is actually a complex impedance and each country has typically designated a different characteristic impedance for their telephone system.
This may not appear to be a problem, but it can have a significant impact on computer, as well as voice, communications. The reason for this impact is related to how a signal is transmitted over a telephone line. A computer using a modem, for example is seen as a signal source because it is transmitting a signal to some telephone network. If the impedance of the modem does not match the impedance of the telephone network, the transmitted signal is reflected back to the modem. In fact, the signal bounces back and forth between the modem and the telephone network until it is attenuated. This signal reflection can cause problems for the telephone network receiving the modem transmission because the reflected signal is superimposed upon the transmitted signal.
When a person is talking on the telephone, an echo may be heard and the conversation is difficult to follow and understand. Similarly, the telephone network receiving the modem transmission has a difficult time hearing the modem signal. This problem is exacerbated by the fact that a modem is transmitting at higher speeds and frequencies. In order to avoid errors, the entity receiving the transmission must ensure that the reflected signal is removed before the transmitted or received signal can be interpreted or decoded. This has an impact on the speed of the transmission and can affect the integrity of the signal.
One solution to the above problem is to physically include more than one impedance on a modem card and cause the modem card, using switching technologies, to select the impedance that matches the characteristic impedance of the telephone network that is connected to the modem. While this solution can work, there are several problems. First, there are many telephone systems, and a great deal of printed circuit board surface area will be required to install all of the necessary impedances. This problem is much more significant with cards conforming to PCMCIA standards, where printed circuit board surface area is very limited. Second, the nominal impedance of the telephone network may be altered because of the length of the transmission line and other factors. Another attempted solution is to place no impedance across the transmission line. Instead, the entire impedance is generated. This solution is unacceptable from a performance standpoint in part because of the signal transients and reflections which are present before the impedance can be generated.
This problem is not limited to telephone networks, but can apply to any situation needing an impedance which is not always constant or where an impedance should be matched. It is desirable to have a circuit or impedance capable of adjusting to different impedances and impedance variations.
It is therefore an object of one embodiment of the present invention to create a termination impedance by adapting a nominal impedance such that the characteristic impedance of a system is terminated and matched.
It is another object of one embodiment of the present invention to modify an impedance which terminates a line.
It is a further object of one embodiment of the present invention to generate an impedance, which when combined with a termination impedance, matches the characteristic impedance of the transmission line.
It is yet another object of one embodiment of the present invention to be able to match the characteristic impedance of a transmission line by generating an impedance.
In summary, one embodiment of the present invention functions to terminate a line by matching the impedance of the source, which may be a telephone network. An effective solution is to physically place a nominal impedance across the transmission line. This nominal impedance may match the characteristic impedance of a system or network. In order to match the characteristic impedances of other systems or telephone networks, an impedance is generated such that when combined with the nominal impedance, a termination impedance is produced which substantially matches the characteristic impedance of the system or network.
The impedance to be generated is easily found because the characteristic impedance and the nominal impedance are both known impedances. The generated impedance is created by sensing the voltage present on the transmission line of the system and converting that voltage to its digital equivalent. The digital equivalent is processed and scaled with a digital signal processor that has an impedance generator implemented in software that alters the digital voltage by some factor. The output of the digital signal processor is converted to an analog voltage and connected to a voltage controlled current source. The voltage which controls the current source creates a current. The voltage of the transmission line divided by the generated current equals the generated impedance. The generated impedance is typically connected across the transmission line in parallel with the nominal impedance. In this manner, the generated impedance, in combination with the nominal impedance, creates a termination impedance which substantially matches the characteristic impedance of the telephone network. Other embodiments of the present invention can be used in any system where a line is terminated or in systems where an impedance must be altered or generated.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.